There are two types of devices for projecting droplets of a liquid, in particular, ink in ink-jet printing systems. One of these types consists in fractionates a pressurized jet into droplets and requires a source of liquid under pressure, a nozzle designed to form a jet, means which can induce high frequency pressure variations in the flow in order to fractionate it, means for directing the droplets and means for recovering and recycling the unused ink. These devices are designed preferably for large plants because they use relatively numerous and complex means and because of the substantial throughput of ink. They are designed for machines such as large computers or very high frequency text processing printers, the cost of which is in line with this type of ink jet device.
The other of these types makes it possible to project droplets at will and generally comprises means for creating an overpressure within an enclosure containing the liquid to be projected and having an opening through which the liquid is expelled whenever the pressure exceeds the force of cohesion of the meniscus formed by the liquid in that opening. Such a device is described, in particular, in U.S. Pat. No. 3,832,579.
This device has a frequency or period of droplet formation which is limited by the speed at which the meniscus can form again after the expulsion of a drop, said period of formation being of the order of 100.times.10.sup.-6 s.
All known ink jet devices use an orifice connected to a reservoir in order to project the droplets. Such devices require droplet tubes in the case of a printer permitting the simultaneous formation of a multiplicity of droplets. A matrix of tubes which can produce this simultaneous projection raises obvious practical problems. In addition, the nozzles through which the ink is projected may clog up, in particular, after a period of nonuse.